Had an idea...
Remebering a data sheet, where you can push up the output voltage of any 78XX linear regulator. Using two diodes on the ground rail to the regulator, anode to cathode in to the regulator. The regulator passes under 1 Amp of the total load, so no problems there. I first saw this two diode string arrangement in an old Harrier cb 2 Amp power supply. On measuring that old psu it was clear the diodes pushed the voltage up. Later i saw the same arrangement in a linear technology data sheet for the 78XX series of regulators. I'm dropping 0.3 Volts now at a shade over 12 Amps. Just the one last picture of the result. Happy with that.

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Just a quick update on the MOT (see posts above) tested with some 40 Amp cable i had laying around. Not enough turns on the secondary to be useful. I'm going to have to drop the cable diameter to get more turns. I've found some high temperature silicon wire on ebay, so probably try that. The cable diameter in the picture is just over 5mm outside diameter. I figure i should be able to achive 30 Amps or close to it it, and a secondary of 15 - 17 Volts AC.

Note on saftey ! The web is full of safe MOT projects, so always do some research on MOT's and always earth the transformer frame, and use a plug in RCD for saftey (residual current device) in the unlikely event of an earth fault this will ensure your safe. If your in any doubt with ability, go and buy a retail power supply. Electricity is safe to work with as long as you follow saftey procedures. Sorry about the saftey rant, but its for good reason.
Hopefully the image posts the right way up. If it didn't sorry about that. I've had to shrink the image to post it.

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The "upside down" version, with a 7912 (not 78) and some 3055's is nice. You probably knew this, just in case...

Juha

Negative rail psu, not much need for that for me.
Dual rail could be handy for testing, but the diagrams schematics on page 1 of this thread work very well, I've built both. If i move on from here in the future, its going to be with the LM723 or very similar regulator. But still multiple series pass transistors. I might make use of the MOT above. The only downside is that does need forced air cooling. Not ideal for continuous on use. But ok for DX sessions etc.

Been meaning to get back to sorting out an old problem. Going back some pages in this thread, you might have seen I was looking for a higher powered higher gain transistor. As an alternative for the circuits in the schematics at the start of the thread.

After much searching I came up with the MJ11015G, back then at the time I used and tested it in the current regulator boost circuit. The problem back then was the transistor was oscillating under load, and the voltage was erratic. Not varying by much, a Volts or two. At the time I gave up on using it for the time being, and using the MJ4502 and TIP2955 and the MJ2955 all PNP types. After thinking back, I had used 2 xbox 360 psu's I converted putting them in series to get 24 Volts. Using this as a makeshift input transformer for testing. Previously it had worked fine with testing circuits using TIP2955's and MJ4502's but thinking back I couldn't come up with a reason for the MJ11015G circuit oscillating.

Since then I'm certain as I can be the problem was an smps input, and not a linear transformer as you would expect. So going back to an old Alti power supply I have, I thought I'd give the MJ11015G another try. I'm glad I did, it runs very well now with a linear transformer input. The main reason for using this transistor was its gain of hfe 1000 and under load it's ability to hold a better voltage, as where lesser gain transistors tend to sagg or drop the voltage some what. Not by a lot, but I think you want to obtain your target voltage as much as possible. Anyway after some heavy load testing, it performance was very good. As I've got more of these, may get some use out of them in the near future. Anyone following, or and experimenting, I'd say give the MJ11015G a go. Couple of pictures of the old Alti power supply that was salvaged.
Best image quality I can upload.

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No, I mean the +12 one, which has the regulation on the bottom side. It has some benefits, large NPNs are more common and cooling and mechanically attaching them is easier as the collectors are at 0 volts.

No, I mean the +12 one, which has the regulation on the bottom side. It has some benefits, large NPNs are more common and cooling and mechanically attaching them is easier as the collectors are at 0 volts.

It looks reasonable enough, i don't use simulation software for what i do. Just try to find as much reference i can to a given circuit. If i had simulation software i would run it in there to be sure of a good outcome. But for all the world it looks ok to me. Yes PNP transistors are more sparse than NPN. But PNP are fairly widley avaliable in TO220, TO3 , TO3P packages. Yes a case collector does really need a case cover if its exterior on the case. Interesting, thanks for posting this.

In the above and other pictures people might have noticed that not much i do is 13.80 Volts, this is only personal preference. All linear psu circuits can, or are just as easily to obtain an output of 13.80 Volts, as opposed to 12 Volts or just above. I prefer to run my gear on 12 Volts or just a tiny bit over. I've noted better RX with weak stations, and things run cooler. I know there's not much in it, it just a preference.

You can obtain 13.80 Volts with almost any linear regulator and series pass trasistors. The data sheets for these regulators shows how you can set up fixed voltages, or a variable voltage should you need that. Both the LM317 and variant packages of it can be easily set up for 13.80 volts. The same for 7812 variants. In the link below there are a couple of LM317 calculators if you wanted to play with resistor combination for that regulator. The 7812 and variants hold loads of options in the data sheets.http://www.reuk.co.uk/wordpress/electri ... alculator/

I've used a largish automotive diode (for alternator, maybe) as a soldering lug, to hold the free ends of the emitter balancing resistors. Just for the convenience, such a diode is easy to bolt or even tap into the heatsink. As a bonus, being parallel the whole pass transistor compound, it gives protection against shorts before the regulator part (the dropped wrench).

I've used a largish automotive diode (for alternator, maybe) as a soldering lug, to hold the free ends of the emitter balancing resistors. Just for the convenience, such a diode is easy to bolt or even tap into the heatsink. As a bonus, being parallel the whole pass transistor compound, it gives protection against shorts before the regulator part (the dropped wrench).

Juha

Diodes make good short circuit protection devices, I've not come across these automotive Diodes, I'll have to look these up. I have 6 Amp rectifier Diodes, but nothing larger. I know an axial lead diode is avaliable in a 10 Amps rated package, beyond that I think it's stud mount Diodes. I usually just use fixed value resistors for the LM317, although I have used a potentiometer a couple of times for part of the circuit. Only once has a transistor failed on me, that went open circuit. The LM317 throttled back on the voltage very quickly, this was good to see, there's no way it could carry the load I had connected. Luckily it was in a test set up scenario. But with connection to radio gear I use a crowbar on the final output.

Strictly linear PSU's sounds a bit like strictly come dancing, Could be an idea for a game show, People build PSU's and bruce forsyth judges them, By sticking his tongue on the mains side of the transformer, Auld clucker I'd watch it

Buick Mackane wrote:Strictly linear PSU's sounds a bit like strictly come dancing, Could be an idea for a game show, People build PSU's and bruce forsyth judges them, By sticking his tongue on the mains side of the transformer, Auld clucker I'd watch it

PMSL I know I know... What can I say, I've no excuses lol but 5 pages in..
It's flexible really, psu repairs etc etc (linear) lol. like thought a few more might have had a go, even alternative circuits, plenty linear power supply circuits out there.

Maybe people do have a go at these ideas and similar circuits. Fare amount of views, not my good looks pulling em in ha ha.
Many must be getting something out of the server converted power supply thread, the number of views there. Bet a lot have saved a bob or two, and still managed to run that shack.

Back on the MOT again, ordered some 12AWG silicone covered cable to wind it a second time. Looking for more turns to get the secondary voltage up higher. The cable came off ebay, really good quality, I was surprised. It's rated to carry up to 45 Amps, that's greater than the previous stuff I wound it with. At a 0.5mm less outside diameter. It claims temperature of 200°C well that's way above what it will ever see in service.

You can see the AC voltage in the picture below, ideally I would have liked a couple more Volts, but I squeezed as many turns on as I could. What I had was 5 meters, what ended up on the core was about 4.4 meters. I figure I'm going to drop at least 1 Volt across a bridge rectifier, unless I can find some high current schottky Diodes, unlikely though. At this voltage I'm going to use a low drop out linear voltage regulator, it only has to provide less than a hundred miliamps or so. The power transistors will carry the current needed.

With 4 x MJ11015 transistors, it should do 15 Amps continuously, and 20 to 25 Amps on a 50 % duty cycle. Nothing happens fast in my world lol. But I should have some positive results in the near future.

Before you consider, (if you do) using a reclaimed MOT (microwave oven transformer) see posts above on earthing the transformer chassis, and testing always using an RCD plug in adaptor. And under no account EVER power up a MOT with its original secondary winding on there. The high voltage secondary is removed to allow for low voltage winding. MOT'S are not super efficient, and do idle at quite high currents, 2 to 3 Amps is typical of an idle MOT primary. And forced air cooling is recommended. If you've not got the knowledge and are not 100 % At working with mains voltages, then please go out and buy your high current power supply needs. There are loads of safe MOT projects across the Web, but please always play safe.

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